Dispersion-shifted fiber

ABSTRACT

A dispersion-shifted fiber includes structure configured to effectively reduce nonlinear optical effects and transmission loss caused by structural mismatching. A core region in the dispersion-shifted fiber includes an inner core and an outer core, which are both glass areas. The inner core is doped with a predetermined amount of fluorine, having an average relative refractive index difference DELTAn1. The outer core is disposed between the inner core and the cladding region and is doped with a predetermined amount of germanium dioxide, having an average relative refractive index difference DELTAn2(DELTAn2&gt;DELTAn1), such that the viscosity ratio between the inner core and the outer core at a drawing temperature is set within a predetermined range, thereby effectively restraining structural mismatching from occurring at the boundary between these glass regions.

TECHNICAL FIELD

The present invention relates to a single-mode optical fiber used as atransmission line in optical communications, in particular, to adispersion-shifted fiber suitable for wavelength-division multiplexing(WDM) transmission.

BACKGROUND ART

Conventionally, in optical communication systems employing a single-modeoptical fiber as their transmission line, light in a 1.3-μm wavelengthband or 1.55-μm wavelength band has often been utilized as signal lightfor communications. Recently, from the viewpoint of lowering thetransmission loss in the transmission line, light in the 1.55-μmwavelength band has been increasingly used. Such a single-mode opticalfiber employed in a transmission line for the light in the 1.55-μmwavelength band (hereinafter referred to as optical fiber for 1.55 μm)has been designed such that its wavelength dispersion (phenomenon inwhich pulse waves spread due to the fact that the propagating speed oflight varies depending on wavelength) of the light in the 1.55-μmwavelength band becomes zero (yielding a dispersion-shifted fiber with azero-dispersion wavelength of 1550 nm). As such a dispersion-shiftedfiber, for example, Japanese Patent Application Laid-Open No. 62-52508proposes a dispersion-shifted fiber having a dual shape core structurein which its core region is constituted by an inner core and an outercore having a refractive index lower than that of the inner core. Also,Japanese Patent Application Laid-Open No. 63-43107 and No. 2-141704 eachpropose a dispersion-shifted fiber having a depressed cladding/dualshape core structure in which its cladding region is constituted by aninner cladding and an outer cladding having a refractive index greaterthan that of the inner cladding. Further, Japanese Patent ApplicationLaid-Open No. 8-304655 and No. 9-33744 each propose a dispersion-shiftedfiber having a ring core structure.

On the other hand, in recent years, the advent of wavelength-divisionmultiplexing transmission and optical amplifiers has enabled therealization of long-haul transmission. Hence, in order to avoidnonlinear optical effects, there has also been proposed adispersion-shifted fiber employing the above-mentioned dual shape corestructure, depressed cladding/dual shape core structure, or the like,with the zero-dispersion wavelength shifted to a wavelength shorter orlonger than the center wavelength of the signal light (Japanese PatentApplication Laid-Open No. 7-168046 and U.S. Pat. No. 5,483,612). Here,the nonlinear optical effects refer to phenomena in which, due tononlinear phenomena such as four-wave mixing (FWM), self-phasemodulation(SPM), cross-phasemodulation (XPM), and the like, signal light pulsesare deformed in proportion to the density in light intensity or thelike. These effects become factors for restricting transmission speed orthe repeater spacing in a relaying transmission system.

In the above-mentioned dispersion-shifted fibers proposed forwavelength-division multiplexing transmission, their zero-dispersionwavelength is set to a value different from the center wavelength of thesignal light, thereby restraining the nonlinear optical effects fromoccurring, or their effective area A_(eff) is elongated so as to reducethe density in light intensity, thereby restraining the nonlinearoptical effects from occurring.

In particular, in the dispersion-shifted fiber shown in theabove-mentioned Japanese Patent Application Laid-Open No. 8-304655 orNo. 9-33744 employing a ring core structure, the dispersion slope ismade smaller, whereas the effective area A_(eff) is made greater, thusrealizing a fiber characteristic suitable for wavelength-divisionmultiplexing transmission.

Here, the effective area A_(eff) is, as disclosed in Japanese PatentApplication Laid-Open No. 8-248251, given by the following expression(1):

 A _(eff)=2π(∫₀ ^(∞) E ² rdr)²/(∫₀ ^(∞) E ⁴ rdr)  (1)

wherein E is the electric field accompanying the propagated light, and ris the radial distance from a core center.

On the other hand, the dispersion slope is defined by the gradient ofthe graph indicating the dispersion characteristic in a predeterminedwavelength band.

DISCLOSURE OF THE INVENTION

Having studied the conventional dispersion-shifted fibers, the inventorshave found the following problems to overcome. Namely, in theabove-mentioned dispersion-shifted fiber comprising a structure foreffectively restraining the nonlinear optical effects from occurring,there is a problem that a dispersion-shifted fiber whose transmissionloss is suppressed to a desired level or lower may not be obtained witha good reproducibility. That is, in the dispersion-shifted fiber ofJapanese Patent Application Laid-Open No. 8-304655 or No. 9-33744employing a ring core structure for suppressing the nonlinear opticaleffects, the thickness of the outer core (difference between the outercore radius and the inner core radius) is very small, i.e., about 1 to 2μm. On the other hand, the average difference between the averagerelative refractive index difference of the average outer core and theaverage relative refractive index difference of the inner core isconsiderably large, i.e., about 1%. For increasing the relativerefractive index difference of the outer core, the amount of GeO₂ addedto the outer core has been increased in general. Increasing the amountof GeO₂ decreases, inversely, the viscosity of the outer core at adrawing temperature during the making of the optical fiber, however.When viewed along a diametrical direction of the optical fiber beingmanufactured, the change in viscosity abruptly occurs within the area ofthe outer core (having a thickness of about 1 to 2 μm). Such an abruptchange in viscosity in the diametrical direction causes, upon drawing ofthe optical fiber, an abrupt change in the tensile force applied theretoin the diametrical direction. The abrupt diametrical change in thedrawing tension thus applied becomes a cause of structural mismatchingor glass defect at the boundary between the inner and outer cores,thereby allowing the resulting optical fiber to increase transmissionloss.

In order to overcome the problems such as those mentioned above, it isan object of the present invention to provide a dispersion-shifted fiberfor WDM transmission suitable for a long-haul submarine cable or thelike, which effectively restrains nonlinear optical effects fromoccurring, effectively suppresses transmission loss caused by structuralmismatching, glass defect, or the average like, and has a structureexcellent in reproducibility.

The dispersion-shifted fiber according to the present invention is atransmission medium (for example, silica(SiO₂) based single-mode (SM)optical fiber), comprising a core region extending along a predeterminedaxis and a cladding region disposed on an outer periphery of the coreregion, for propagating signal light in a 1.55-μm wavelength band (i.e.,at least one signal light component having a center wavelength withinthe wavelength range of 1500 nm to 1600 nm). In this dispersion-shiftedfiber, the core region, includes, an inner core which is a glass area,doped with a predetermined amount of fluorine (F), having a firstaverage relative refractive index difference Δn₁ with respect to apredetermined region (reference region) of the cladding region; and anouter core which is a glass area doped with a predetermined amount ofgermanium oxide (GeO₂) and disposed between the inner core and thecladding region, having a second average relative refractive indexdifference Δn₂ greater than the first average relative refractive indexdifference Δn₁ with respect to the predetermined region of the claddingregion. Here, the predetermined region is defined by a single layer inthe case that the cladding region is constituted by the single layer,and is also defined by the outermost layer in the case that the claddingregion is constituted by a plurality of layers.

In the dispersion-shifted fiber according to the present invention, bothgermanium dioxide (GeO₂) and fluorine (F), respectively, are added tothe inner and outer cores to reduce the viscosity at a drawingtemperature; The GeO₂ acts to increase the refractive index of the glassarea doped therewith, while F acts to lower the refractive index of theglass area doped therewith. Hence, when the inner and outer cores aredoped with F and GeO₂, respectively, while the viscosity differencebetween the inner and outer cores at a drawing temperature is keptsmaller as compared with the case where the inner core is not doped withF, a sufficient average relative refractive index difference can beobtained between these glass areas. As a consequence, the structuralmismatching or glass defect at the boundary between the inner and outercores can be effectively restrained from occurring. Here, the drawingtemperature is defined by a surface temperature of an optical fiberpreform which is sufficiently heated for drawing.

In the dispersion-shifted fiber according to the present invention, adecrement Δn_(f) in the first average relative refractive indexdifference caused by the F doping in the inner core and an incrementΔn_(g)in the second average relative refractive index difference causedby the GeO₂ doping in the outer core have the following relationship:

0.05·Δn _(g) ≦Δn _(f)≦0.07·Δn _(g).

When the above-mentioned relationship is satisfied, the viscositydifference between the inner and outer cores at a drawing temperaturecan be controlled so as to become smaller. Also, the structuralmismatching, glass defect, and the like can be further restrained fromoccurring at the boundary between the inner core and outer core.

In the dispersion-shifted fiber according to the present invention, itis preferred that the average relative refractive index difference inthe inner core be smaller than the refractive index of the claddingregion.

When the first average relative refractive index difference Δn₁ withrespect to the cladding region is set to a negative value, a desiredfiber characteristic can be realized without extremely increasing thesecond average relative refractive index difference Δn₂ (Δn₂>Δn₁) withrespect to the cladding region. This can reduce the amount of GeO₂ addedto the outer core, and thus is more preferable from the viewpoint ofrestraining the nonlinear optical effects from occurring.

Further, a depressed cladding structure, which is constituted by aninner cladding provided around the outer periphery of the outer core andan outer cladding provided around the outer periphery of the innercladding, can be applied to the cladding region. In this case, since thecladding region comprises a plurality of layer regions, a predeterminedregion for defining the average relative refractive index difference ineach region is the outer cladding. In this cladding region, the innercladding is doped with a predetermined amount of fluorine and has athird average relative refractive index difference Δn₃ with respect tothe outer cladding.

Additionally, in the dispersion-shifted fiber having the depressedcladding structure, a decrement Δn_(f1) in the first average relativerefractive index difference caused by the F doping in the inner core, adecrement Δn_(f2) in the third average relative refractive indexdifference caused by the F doping in the inner cladding and an incrementΔ n_(g) in the second average relative refractive index differencecaused by the GeO₂ doping in the outer core have the followingrelationships:

0.05·Δn _(g) ≦Δn _(f1)≦0.7·Δn _(g)

0.05·Δn _(g) ≦Δn _(f2)≦0.7·Δn _(g).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a view showing a typical cross-sectional structure of adispersion-shifted fiber according to the present invention, whereas

FIG. 1B is a view showing a refractive index profile of thedispersion-shifted fiber shown in FIG. 1A;

FIG. 2 is a view showing a refractive index profile of a conventionaldispersion-shifted fiber employing a dual shape core structure;

FIG. 3 is a graph showing a relationship between the ratio(Δn_(f)/Δn_(g)) of the change Δn_(f) in average refractive index of theinner core caused by the F doping to the change Δn_(g) in averagerefractive index of the outer core caused by the GeO₂ doping and theviscosity ratio at a drawing temperature;

FIG. 4A is a view showing a cross-sectional structure of thedispersion-shifted fiber according to Embodiment 1 of the presentinvention, whereas

FIG. 4B is a view showing a refractive index profile of thedispersion-shifted fiber shown in FIG. 4A;

FIG. 5A is a view showing a cross-sectional structure of thedispersion-shifted fiber according to Embodiment 2 of the presentinvention, whereas

FIG. 5B is a view showing a refractive index profile of thedispersion-shifted fiber shown in FIG. 5A;

FIG. 6 is a table listing various characteristics of thedispersion-shifted fibers shown in FIGS. 4A and 4B and FIGS. 5A and 5B;and

FIG. 7A is a view showing a cross-sectional structure of thedispersion-shifted fiber according to Embodiment 3 of the presentinvention, whereas

FIG. 7B is a view showing a refractive index profile of thedispersion-shifted fiber shown in FIG. 7A.

BEST MODES FOR CARRYING OUT THE INVENTION

In the following, embodiments of the dispersion-shifted fiber accordingto the present invention will be explained with reference to FIGS. 1A,1B, 2 and 3, 4A to 5B, 6, 7A and 7B. In the explanation of the drawings,constituents identical to each other will be referred to with numeralsor letters identical to each other without repeating their overlappingdescriptions.

FIG. 1A is a view showing a cross-sectional structure of adispersion-shifted fiber (silica SiO₂ based single-mode (SM), opticalfiber) according to the present invention, whereas FIG. 1B is a viewshowing a refractive index profile of the dispersion-shifted fiber shownin FIG. 1A. As shown in FIG. 1A, this dispersion-shifted fiber 1 is asingle-mode optical fiber mainly composed of silica glass; comprises acore region and a cladding region 12, disposed on the outer periphery ofthe core region, having a predetermined refractive index; and functionsas a transmission medium for propagating signal light in a 1.55-μmwavelength band (at least one signal light component having a centerwavelength within the wavelength range of 1500 nm to 1600 nm). Inparticular, employed as the structure of the core region is a ring corestructure comprising, an inner core 10 (having an outer diameter a)which is a glass area, doped with a predetermined amount of fluorine(F), having an average relative refractive index difference Δn₁ withrespect to the cladding region 12; and an outer core 11 (having an outerdiameter b>a) which is a glass area, disposed on the outer periphery ofthe inner core 10 and doped with a predetermined amount of germaniumoxide (GeO₂), having an average relative refractive index difference Δn₂(>Δn₁) with respect to the cladding region 12. Here, the cladding region12 may also be of a depressed cladding structure comprising at least twoglass areas having refractive indexes different from each other.

The refractive index profile 150 of FIG. 1B indicates the refractiveindex of each location along the line L1 intersecting with a center O₁in a cross section (plane orthogonal to the advancing direction of thepropagating light) of the dispersion-shifted fiber 1 in conformity toFIG. 1A. In the refractive index profile 150, areas 100, 101, and 102correspond to locations, on the line Li, of the inner core 10, outercore 11, and cladding region 12, respectively.

The above-mentioned average relative refractive index difference valuesΔn₁ and Δn₂ are given by:

Δn ₁=(n ₁ −n _(cd))/n _(cd)

Δn ₂=(n ₂ −n _(cd))/n _(cd)

wherein

n₁ is the average refractive index of the inner core 10;

n₂ is the average refractive index of the outer core 11; and

n_(cd) is the average refractive index of the cladding region 12 as apredetermined region (refractive index of the outermost cladding in thecase of a depressed cladding structure).

In this specification, these values are expressed in terms ofpercentage. The refractive indexes in each equation may be arranged inany order. As a consequence, in this specification, the glass area wherethe average of relative refractive index differences with respect to thecladding region 12 is a negative value refers to a glass area having anaverage refractive index lower than that of the cladding region 12.Also, the average refractive index refers to the average of refractiveindexes at their respective locations in a predetermined glass area whena cross section perpendicular to the advancing direction of signal lightin the dispersion-shifted fiber 1 is observed.

The relationship between the amount of addition of germanium to a silicaglass and its refractive index can be obtained from James W. Fleming,“Dispersion in GeO₂-SiO₂ glasses,” (APPLIED OPTICS, Vol. 24, No. 24,Dec. 15, 1984, pp. 4486-4493); whereas the relationship between theamount of F added to a silica glass and its refractive index can beobtained from James W. Fleming et al., “Refractive index dispersion andrelated properties in fluorine doped silica,” (APPLIED OPTICS, Vol. 23,No. 19, Oct. 1, 1983, pp. 3102-3104). The dispersion-shifted fiber 1according to the present invention can be obtained by heating anddrawing an optical fiber preform manufactured according to well-knownOVD technique or MCVD technique or the like.

In particular, in the dispersion-shifted fiber 1 according to thepresent invention, the decrement Δn_(f) in the average relativerefractive index difference caused by the F doping in the inner core 10and the increment Δn_(g) in the average relative refractive indexdifference caused by the GeO₂ doping in the outer core 11 satisfy thefollowing relationship:

0.05·Δn _(g) ≦Δn _(f)≦0.7·Δn _(g).

It has been known that, in the case where viscosity mismatching betweenindividual glass areas is large in a cross section of an optical fiber,the transmission loss caused by structural mismatching (structuralmismatching loss) would increase (the Institute of Electronics,Information and Communication Engineers, Electronics Society Convention1995, C-232). FIG. 2 is a view showing a refractive index profile of theconventional dispersion-shifted fiber employing a dual shape corestructure shown in the publication mentioned above. In this refractiveindex profile, of an area 110 (corresponding to the individual locationsalong a diametrical direction of the whole core region), areas 111, 112,and 120 correspond to locations, along the diametrical direction, of theinner core, outer core, and cladding region.

Here, in order to suppress the structural mismatching loss to the extentsimilar to that in the dispersion-shifted fiber shown in FIG. 2, it isnecessary for the viscosity mismatching (difference in viscosity at adrawing temperature) between the inner core 10 and outer core 11 to beconsistent with that in the dispersion-shifted fiber of FIG. 2. In thedispersion-shifted fiber of FIG. 2, both inner and outer cores are dopedwith GeO₂, and the difference between the average relative refractiveindex difference of the average inner core and the average relativerefractive index difference of the outer core is about 0.75%, which isequivalent to a viscosity ratio of about 4:1.

On the other hand, as shown in FIG. 3, the viscosity ratio between anSiO₂ glass doped with GeO₂ and an SiO₂ glass doped with F at apredetermined drawing temperature (defined by the surface temperature ofan optical fiber preform to be heated) varies as the respective dopingamounts of F and GeO₂ change. This relationship is disclosed in P. K.Bachman, et al., “Stress in optical waveguides 2: Fibers,”(APPLIEDOPTICS, Vol. 26, No. 7, Apr. 1, 1987).

Namely, FIG. 3 shows, when the viscosity ratio between the inner core 10and the outer core 11 is calculated according to the above-mentionedrelationship in the dispersion-shifted fiber 1, how the viscosity ratiochanges according to the ratio (Δn_(f)/Δn_(g)) between decrement Δn_(f)in the average relative refractive index difference caused by the Fdoping in the inner core 10 and the increment Δn_(g) in the averagerelative refractive index difference caused by the GeO₂ doping in theouter core 11. The decrement Δn_(f) and increment Δn_(g) are eachrepresented by an average relative refractive index difference withrespect to the cladding 12 (wherein the changes Δn_(f) and Δn_(g) areboth scalar quantities). Also, FIG. 3 shows a typical value of thedispersion-shifted fiber 1, using the increment Δn_(g) caused by theGeO₂ doping into the outer core 11 as a parameter (Δn_(g)=0.8%, 1.0%,and 1.2%).

As can be seen from FIG. 3, when An_(f)/Δn_(g) is within the range of0.05 to 0.70, the viscosity ratio between the inner core 10 and theouter core 11 becomes 4 or less, thereby the structural mismatching losscan be suppressed to the extent similar to that of thedispersion-shifted fiber of FIG. 2 or less.

In the dispersion-shifted fiber according to the present invention, itis more preferred that the average refractive index of the inner core 10be smaller than the refractive index of the cladding region 12. It isdue to the fact that, when the relative refractive index difference Δn₁with respect to the cladding region 12 is set to a negative value, adesired fiber characteristic can be realized without extremelyincreasing the average relative refractive index difference Δn₂ (>Δn₁)of the outer core 11 with respect to the cladding region 12. In otherwords, while attaining a desired fiber characteristic, the amount ofGeO₂ added to the outer core 11 can be reduced, thus making it possibleto effectively reduce nonlinear optical effects from occurring.

Embodiment 1

The dispersion-shifted fiber according to Embodiment 1 of the presentinvention will now be explained with reference to FIGS. 4A, 4B, and 6.

The cross-sectional structure of the dispersion shifted fiber 2according to Embodiment 1 shown in FIG. 4A is basically the same as thatof the dispersion-shifted fiber 1 shown in FIG. 1A; and comprises aninner core 20 (corresponding to the inner core 10) having an outerdiameter a, an outer core 21 (corresponding to the outer core 11) havingan outer diameter b, and a cladding region 22 (corresponding to thecladding region 12). The refractive index profile 250 of FIG. 4B, as inthe case of FIG. 1B, indicates the refractive index of each location onthe line L2 intersecting with a center O₂ in a cross section (planeorthogonal to the advancing direction of the propagating light) of thedispersion-shifted fiber 2 in conformity to FIG. 4A. In the refractiveindex profile 250, areas 200, 201, and 202 correspond to locations, onthe line L2, of the inner core 20, outer core 21, and cladding region22, respectively.

In Embodiment 1, the outer diameter b of the outer core 21 is 7.5 μm,and the outer diameter ratio Ra (=a/b) between the inner core 20 and theouter core 21 is 0.65. The inner core 20 is doped with F, whereas theouter core 21 is doped with GeO₂l thereby the average relativerefractive index difference Δn₁ of the inner core 20 and the averagerelative refractive index difference Δn₂ of the outer core 21, which aregiven by the above-mentioned defining equations, are set to −0.40% and+1.20%, respectively.

The table of FIG. 6 (Embodiment 1) shows fiber characteristics of thusdesigned dispersion-shifted fiber 2 with respect to signal light at awavelength of 1550 nm. The inventors have confirmed that thetransmission loss of thus obtained dispersion-shifted fiber 2 withrespect to signal light at a wave length of 1550 nm is small, i.e., 0.22dB/km. Also, in the illustrated embodiment, the dispersion-shifted fiber2 has a zero-dispersion wave length of 1580 nm, a dispersion slope of0.088 ps/nm²/km, and an effective area of 86 μm², thereby realizingfiber characteristics suitable for WDM transmission.

The table of FIG. 6 also lists nonlinear refractive index N2. It is dueto the fact that, as the advent of optical amplifiers has enabledtechniques for wavelength-division multiplexing long-haul opticaltransmission, the distortion in signal light pulses caused by nonlinearoptical effects such as four-wave mixing has become a critical factorrestricting the transmission distance and transmission bit rate, whichfactor is not negligible in the making of the dispersion-shifted fiberaccording to the present invention.

The nonlinear optical effects causing the distortion in signal lightpulses have been known to increase in proportion to the optical powerdensity (density of signal light intensity at a predetermined locationin an SM optical fiber) and the nonlinear refractive index of theoptical fiber, which is an optical transmission medium. Consequently, inan optical transmission system employing an optical amplifier, thedistortion in signal light pulses caused by nonlinear optical effectswhich have not been problematic in the conventional optical transmissionsystem employing no optical amplifier is not negligible anymore as thesignal light intensity increases.

Here, the refractive index N of a medium under strong light variesdepending on light intensity as mentioned above. Consequently, thelowest-order effect with respect to the refractive index N isrepresented by:

N=N0+N2·P

wherein

N0 is the refractive index with respect to linear polarization;

N2 is the nonlinear refractive index with respect to the third-ordernonlinear polarization;

P is the optical power; and

A_(eff) is the effective area.

Namely, under strong light, the refractive index N of the medium isgiven by the sum of the normal value N0 and the increment proportionalto the square of the photoelectric field amplitude of light E. Inparticular, the constant of proportionality N2 (unit: m²/W) in thesecond term is known as nonlinear refractive index. Since the distortionin signal light pulses is mainly influenced by, of the nonlinearrefractive index, the second-order nonlinear refractive index, thenonlinear refractive index in this specification mainly refers to thesecond-order nonlinear refractive index.

Embodiment 2

The dispersion-shifted fiber according to Embodiment 2 of the presentinvention will now be explained with reference to FIGS. 5A, 5B, and 6.

The cross-sectional structure of the dispersion-shifted fiber 3according to Embodiment 2 shown in FIG. 5A is basically the same as thatof the dispersion-shifted fiber 1 shown in FIG. 1A; and comprises aninner core 30 (corresponding to the inner core 10) having an outerdiameter a, an outer core 31 (corresponding to the outer core 11) havingan outer diameter b, and a cladding region 32 (corresponding to thecladding region 12). The refractive index profile 350 of FIG. 5B, as inthe case of FIG. 1B, indicates the refractive index of each location onthe line L3 intersecting with a center O₃ in a cross section (planeorthogonal to the advancing direction of the propagating light) of thedispersion-shifted fiber 3 in conformity to FIG. 5A. In the refractiveindex profile 350, areas 300, 301, and 302 correspond to locations, onthe line L3, of the inner core 30, outer core 31, and cladding region32, respectively.

In Embodiment 2, the outer diameter b of the outer core 21 is 7.1 μm,and the outer diameter ratio Ra (=a/b) between the inner core 30 and theouter core 31 is 0.60. The inner core 30 is doped with F, whereas theouter core 31 is doped with GeO₂, thereby the average relativerefractive index difference Δn₁ of the inner core 30 and the averagerelative refractive index difference Δn₂ of the outer core 31, which aregiven by the above-mentioned defining equations, are set to −0.60% and+1.00%, respectively.

Thus, in the dispersion-shifted fiber 3 according to Embodiment 2, theamount of F added to the inner core 30 is made greater than that inEmbodiment 1 (thus changing the average relative refractive indexdifference Δn₁ from −0.40% to −0.60%), thereby reducing the amount ofGeO₂ added to the outer core 31, without changing the difference betweenthe average relative refractive index difference of the inner core 30and the average relative refractive index difference of the outer core31.

The table of FIG. 6 (Embodiment 2) shows fiber characteristics of thusdesigned dispersion-shifted fiber 3 with respect to signal light at awavelength of 1550 nm. As can be seen from this table, thedispersion-shifted fiber 3 according to Embodiment 2 can lower theaverage relative refractive index difference Δn₂ of the outer core 31 to1.00%, while maintaining, substantially as with Embodiment 1 mentionedabove, a zero-dispersion wavelength of 1580 nm, a dispersion slope of0.086 ps/nm²/km, and an effective cross-sectional area of 83 μm². Thetransmission loss of the dispersion-shifted fiber 3 according toEmbodiment 2 with respect to signal light at a wavelength of 1550 nm is0.21 dB/km, which is smaller than that in Embodiment 1 mentioned above.

Embodiment 3

The dispersion-shifted fiber according to Embodiment 3 of the presentinvention will now be explained with reference to FIGS. 7A and 7B.

The cross-sectional structure of the dispersion-shifted fiber 4according to Embodiment 3 shown in FIG. 7A comprises an inner core 40having an outer diameter a, an outer core 41 having an outer diameter b,an inner cladding 42 having an outerside diameter c, and an outercladding 43. The refractive index profile 450 of FIG. 7B, as in the caseof FIG. 1B, indicates the refractive index of each location on the lineL4 intersecting with a center O_(O) in a cross section (plane orthogonalto the advancing direction of the propagating light) of thedispersion-shifted fiber 4 in conformity to FIG. 7A. In the refractiveindex profile 450, areas 400, 401, 403 and 404 correspond to locations,on the line L4, of the inner core 40, outer core 41, inner cladding 42,and outer cladding 43, respectively.

In Embodiment 3, the outer diameter a of the inner core 40 is 4.4 μm,the outer diameter b of the outer core 41 is 7.9 μm, the outer diameterc of the inner cladding 42 is 13.9 μm. The inner core 4 and the innercladding 42 are doped with F, whereas the outer core 41 is doped withGeO₂, thereby the average relative refractive index difference Δn₁ ofthe inner core 40, the average relative refractive index differenceΔn₂of the outer core 41, and the average relative refractive indexdifference Δn₃ of the inner cladding 42, which are given by theabove-mentioned defining equations, are set to −0.50%, +1.00%, and−0.20%, respectively.

The above-mentioned average relative refractive index difference valuesΔn₁, Δn₂, and Δn₃ are given by:

Δn ₁=(n ₁ −n _(cd))/n _(cd)

Δn ₂=(n ₂ −n _(cd))/n _(cd)

Δn ₃=(n ₃ −n _(cd))/n _(cd)

wherein

n₁ is the average refractive index of the inner core 40;

n₂ is the average refractive index of the outer core 41;

n₃ is the average refractive index of the inner core 42; and

n_(cd) is the average refractive index of the outer cladding 43.

Thus designed dispersion-shifted fiber of Embodiment 3 has azero-dispersion wavelength of 1590 nm, and has, as characteristics withrespect to light of 1550 nm, a dispersion slope of 0.070 ps/nm₂/km, aneffective core sectional area of 83 μm², a transmission loss of 0.21dB/km, a cutoff wavelength of 1.4 μm at 2 m length, a mode fielddiameter (MFD) of 7.4 μm, a nonlinear refractive index(N2) of 3.7×10⁻²⁰m²/W, and a bending loss of 5.7 dB/m at a bending diameter of 20 mm.

Further, in Embodiment 3 having a depressed cladding structure asdescribed above, a decrement Δn_(f1) in the first average relativerefractive index difference caused by the F doping in the inner core, adecrement Δn_(f2) in the third average relative refractive indexdifference caused by the F doping in the inner cladding and an incrementΔn_(g) in the second average relative refractive index difference causedby the GeO₂ doping in the outer core satisfy the followingrelationships:

 0.05·Δn _(g) ≦Δn _(f1)≦0.7·Δn _(g)

0.05·Δn _(g) ≦Δn _(f2)≦0.7·Δn _(g).

INDUSTRIAL APPLICABILITY

As explained in detail in the foregoing, since the inner core is dopedwith a predetermined amount of F, and the outer core is doped with apredetermined amount of GeO₂, such that the viscosity ratio between theindividual glass regions at a drawing temperature is set within apredetermined range, the present invention can securely reduce thestructural mismatching, glass defect, and the like at the boundarybetween the inner core and the outer core, thereby effectivelyrestraining transmission loss from increasing due to such a problem.

Also, since the average refractive index of the inner core is set lowerthan the refractive index of the cladding region, the amount of GeO₂added to the outer core can be lowered, thereby the transmission lossincreasing depending on the amount of GeO₂ can be further reduced.

What is claimed is:
 1. A dispersion-shifted fiber for propagating signallight in a 1.55-μm wavelength band, said dispersion-shifted fibercomprising: a core region extending along a predetermined referenceaxis; and a cladding region disposed on the outer periphery of said coreregion, said core region comprising: an inner core doped with apredetermined amount of fluorine and having a first average relativerefractive index difference Δn₁ with respect to a predetermined regionof said cladding region; and an outer core provided between said innercore and said cladding region, said outer core doped with apredetermined amount of germanium dioxide and having a second averagerelative refractive index difference Δn₂ greater than said first averagerelative refractive index difference with respect to said predeterminedregion of said cladding region, wherein a decrement An_(f) in said firstaverage relative refractive index difference caused by the fluorinedoping in said inner core and an increment Δn_(g) in said second averagerelative refractive index difference caused by the germanium dioxidedoping in said outer core have the following relationship: 0.05·Δn _(g)≦n _(f)≦0.7·Δn _(g).
 2. A dispersion-shifted fiber according to claim 1,wherein said inner core has an average refractive index smaller than therefractive index of said cladding region.
 3. A dispersion-shifted fiberaccording to claim 1, wherein said cladding region comprises an innercladding provided around the periphery of said outer core, said innercladding doped with a predetermined amount of fluorine and having athird average relative refractive index difference Δn₃ with respect tosaid predetermined region of said cladding region; and an outer claddingas said predetermined region of said cladding region, provided aroundthe outer periphery of said inner cladding.
 4. A dispersion-shiftedfiber according to claim 3, wherein a decrement An_(f1), in said firstaverage relative refractive index difference caused by the fluorinedoping in said inner core, a decrement Δn_(f2) in said third averagerelative refractive index difference caused by the fluorine doping insaid inner cladding, and an increment Δn_(g) in said second averagerelative refractive index difference caused by the germanium dioxidedoping in said outer core have the following relationships: 0.05·Δn _(g)≦Δn _(f1)≦0.7·Δn _(g) 0.05·Δn _(g) ≦Δn _(f2)≦0.7·Δn _(g).